1. Field of the Invention
The invention relates to wavelength division optical multiplexing and, more particularly, to wavelength division demultiplexers and multiplexers including arrays of optical filtering elements such as Bragg gratings, that form optical filters having characteristic wavelength passbands.
2. Description of the Related Art
Wavelength-division multiplexing is a valuable technique for increasing the information-carrying capacity of an optical communication network. In a multiplexed network, it is essential to provide wavelength-selective components that can separate a particular wavelength channel from a group of multiplexed channels. Bragg gratings, elements comprising regions of refractive index perturbations which reflect particular wavelengths, are advantageously used to provide spectral selectivity. They are readily made with passbands that match desired channel widths, and they are readily integrated with optical waveguides such as optical fibers, planar, and channel waveguides. Current channel-selective filters using Bragg gratings can suffer from relatively high coupling losses; others are difficult to manufacture because they must be made to extremely close tolerances.
For example, one known filter configuration includes an input fiber coupled to a fiber-based Bragg grating through a directional optical coupler. Light reflected from the grating is coupled to an output waveguide through the same coupler. In this manner, the narrow reflection band of the grating is effectively transformed into a transmission band for the filter as a whole. However, this arrangement suffers 6 dB of coupling loss, to which is added the intrinsic loss in the directional coupler.
Another example is a filter described in K. O. Hill et al., "Narrow-Bandwidth Optical Waveguide Transmission Filters," Electronics Letters 23 (1987) pp. 465-466. This filter consists of a loop of single-mode optical fiber which includes a fused-taper directional coupler at the point where the loop branches off from the straight part of the fiber. A Bragg grating is situated symmetrically midway in the fiber loop. This filter can be made to have relatively low loss. However, this filter is difficult to manufacture because exact placement of the Bragg grating is required in order to achieve the desired transmission characteristics. Bilodeau et al., "High-Return-Loss Narrowbond All-Fiber Bandpass Bragg Transmission Filter," IEEE Photonics Technology Letters, Vol. 6, No. 1, (1994), pp. 80-82 describes a design modification of the Hill et al. filter.
Compound Bragg reflection filters made in planar waveguides are described in C. H. Henry et al., "Compound Bragg Reelection Filters Made by Spatial Frequency Doubling Lithography," J. Lightwave Technol. 7 (1989) pp. 1379-1385. As reported therein, a filter having desirable spectral properties can be made by forming a Bragg grating having many, e.g., 15, sections, each with a different spatial period. To avoid phase shifts near 90.degree. between sections, the sections are made contiguous, resulting in a grating period that is piecewise constant, but discontinuously varied.
Accordingly, there is a need in the art for wavelength division multiplexing elements which are simple to manufacture and which are readily formed from optical filtering elements which separate a particular wavelength channel from a group of multiplexed channels. There is a further need in the art for wavelength division optical multiplexing elements which are insensitive to the polarization of incident radiation.